During respiration of live beings significant quantities of carbon dioxide is released. This CO2 can have asphyxiating effect when present in high concentrations.
Management of the harmful gases present in a variety of concentrations is traditionally carried out by elimination of a volume of returned air and the intake of an equivalent amount of fresh air from the exterior. Fresh air from the exterior requires several treatments of high power consumption treatment in order to gain similar properties to the volume of rejected air which have undergone many processes such as: heated/cooled, humidified/dehumidified.
Closed buildings or closed environments are installations wherein equipment and environmental control for occupants is assured only by a ventilation system. The efficiency of the ventilation system depends on the constant adjustment of airflow to different zones of the building and it is not possible to open windows or doors leading to the exterior of the building.
The maintenance of air quality to ensure human respiration and activity can only be assured by the elimination of carbon dioxide, dust and other noxious gases. Symptoms due to the exposure to high concentrations of carbon dioxide are well known and thus affirm a need for a proper ventilation system designed for closed environments. As such, the aerospace industry has previously proposed several biological and physico-chemical devices to resolve these problems.
One method of controlling carbon dioxide is to use a bio-regenerative system involving green plants known for their photosynthetic capacities. Such a method consists of circulating into a greenhouse, or a system including plants, stale air produced by human respiration, to convert the carbon dioxide into oxygen. This method permits to affix atom of carbons from carbon dioxide to generate an additional mass of green plants able to treat additional volumes of gas. Such a system is described in U.S. Pat. No. 5,005,787. Limitations of such a method include the amount of space required for a greenhouse as well as the amount of energy needed to stimulate the growth of green plants. Furthermore, the active element of this system, green plants, requires special care in order to avoid degradation and/or death.
Another method consists of chemically eliminating carbon dioxide by using lithium hydroxide filters. These filters are used for the environment control systems (ECS) of spacecraft, space suits and submarines. The elimination of carbon dioxide with this method involves two reactions.
The first reaction consists of forming lithium hydroxide monohydrate from lithium hydroxide and water.
The second reaction implies the reaction between lithium hydroxide monohydrate and carbon dioxide to form lithium carbonate and water.
The reaction between carbon dioxide and lithium hydroxide is spontaneous only when in contact with humid air.
Generally, the use of such systems is based on canisters, which are similar to filters, and which contain a limited quantity of solid lithium hydroxide needed to treat a fixed volume of carbon dioxide (CO2). The efficiency of such a system is not questioned, however the problematic of the short life span of these canisters limits the volume of air capable of being treated.
Other systems have also been developed around CO2 sorption with solid phase amines. In such systems, primary and secondary alkanolamines react with dissolved CO2 in a two step sequence. The first sequence involves the formation of a zwiterrion. In a second sequence the zwiterrion transfers a proton to an unionised amine, forming the corresponding carbamate.R1R2NH+CO2(aq)→R1R2NH+CO2−R1R2NH+R1R2NH+CO2−→R1R2NH++R1R2NCO2−
The reaction of tertiary amines with CO2 proceeds by the formation of a protonated amine and a bicarbonate anion.R1R2R3N+H2O+CO2(aq)→R1R2R3NH++HCO3−
Three commonly used alkanolamine CO2 sorbents are monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA). The solid phase amine CO2 sorbents work by analogous reactions between airborne CO2, water vapour, and the amine bearing the polymer functional groups (U.S. Pat. No. 6,364,938). Some CO2 removal systems, used in aerospace, are based on Amberlite XAD-7 beds with a polyethyleneimine coat (U.S. Pat. No. 4,046,529).
These systems, based on the principle of elimination, contain two chambers working in parallel to treat air. Each of these two chambers works for definite periods before being regenerated. In other words, as one chamber is being used, the second chamber is in a regenerating mode. A problem associated with the two chamber operating mode is the high consumption of energy since it operates in a vacuum (closed environment). In certain cases, these systems require the input of energy in the form of heat to be operable. This is a major drawback to the use of such a system in a terrestrial environment, and particularly for an enclosed building.
Another chemical method of eliminating carbon dioxide is by using metal oxides. This method has vast applications including treating cryogenic gases and autonomous mobile space units. The most recent type of air treatment system used in spatial mobile units is based on silver oxide. The reaction of carbon dioxide with silver oxide, performed in the presence of water, is carried out to produce silver carbonate.Ag2O+CO2→Ag2CO3
The reaction can also be reversible at high temperatures:Ag2CO3→Ag2O+CO2(220° C.)
The reversibility of such a process is important when re-using the active element of the system. However, upon several regenerations, the structure has a tendency to breakdown.
Another carbon dioxide elimination technology of physical type consists of using a molecular sieve in conjunction with porous materials to capture carbon dioxide. Molecular sieves, which are made of zeolites composed of silica and alumina, exhibit a high porosity and a regular pore dimension due to their crystalline structure. In such case, carbon dioxide sets in the pores of the crystal lattice and remains captive until the molecular sieve is regenerated. These molecular sieves are equipped with a device adapted to pre-treat the air by means of a similar molecular sieve capable of adsorbing the water molecules. This device is essential since the presence of water vapour inhibits CO2 absorption in this type of process. The sensitivity of the process to humidity present in air as well as the need to thermally regenerate or vacuum the sieve constitute two significant drawbacks if it were to be applied in a ventilation system of a building.
Also known in the prior art, there is CA 2291785 in the name of the applicant which discloses generally the use of carbonic anhydrase in a packed tower-type bioreactor. There is also U.S. Pat. No. 6,143,556 which also generally discloses the use of the enzyme, carbonic anhydrase.
Thus there is still presently a need for a ventilation system that would efficiently and easily allows carbon dioxide contained in recycled stale air to be greatly reduced or even completely removed from the air.